Mirror lenses were a fad during the 1970's. They allowed a powerful zoom lens with a short barrel. This made the lenses more managable. But on the down side, these days all mirror lenses are manual only, so a bit of camera skill is required.
All consided, I like this lens and the fiddle of having to use a manual light meter. It took my back to the days of film cameras, when calculating the exposure was normal. People who do not understand ISO, f-stop, shutter speed and focal length will find this type of lens very challenging.